Process for the epoxidation of olefins

ABSTRACT

The present invention relates to a process for the catalytic epoxidation of olefins in which in one reaction stage the olefin is reacted with aqueous hydrogen peroxide in an organic, water-miscible solvent in the presence of a titanium silicalite catalyst, wherein an exit gas stream is obtained which contains olefin oxide, unreacted olefin and oxygen and this exit gas stream is brought into contact in an absorption unit with the same solvent as used in the reaction stage and a solvent stream loaded with olefin and olefin oxide is drawn off from the absorption unit and an exit gas stream containing oxygen is discharged.

[0001] The present invention relates to a process for the epoxidation ofolefins, in which the exit gas stream leaving the reactor is furtherworked up.

[0002] 1. Prior Art

[0003] It is known from EP A 100 118 that propene may be reacted withhydrogen peroxide to yield propene oxide if titanium silicalite is usedas the catalyst. A secondary reaction which always occurs to a slightextent on the titanium silicalite catalyst is the decomposition ofhydrogen peroxide to form molecular oxygen. If it is to be possible tooperate the epoxidation process safely on an industrial scale, theoxygen formed must be removed from the reaction system. This is mostsimply achieved by discharging it with a propene exit gas stream. Such aprocess is known from EP A 659 473. The process does, however, have thedisadvantage that considerable quantities of propene and propene oxideare lost together with the oxygen.

[0004] The object of the present invention is accordingly to provide aprocess for the epoxidation of olefins with which higher product yieldsmay be achieved.

[0005] 2. Subject Matter of the Invention

[0006] This object is achieved by a process for the catalyticepoxidation of olefins in which in one reaction stage the olefin isreacted with aqueous hydrogen peroxide in an organic, water-misciblesolvent in the presence of a titanium silicalite catalyst, wherein anexit gas stream is obtained which contains olefin oxide, unreactedolefin and oxygen and this exit gas stream is brought into contact in anabsorption unit with the same solvent as used in the reaction stage anda solvent stream loaded with olefin and olefin oxide is drawn off fromthe absorption unit and an exit gas stream containing oxygen isdischarged.

[0007] 3. Description of the Invention

[0008] It has now been found that the losses of olefin and olefin oxidewhich occur on discharge of the exit gas stream containing oxygen duringthe epoxidation of olefin with hydrogen peroxide and a titaniumsilicalite catalyst may be reduced in a simple manner by absorbing themajority of the olefin oxide, olefin and optionally the correspondingalkane with the solvent used for the epoxidation, discharging the oxygenand either returning the solvent stream loaded with olefin oxide andolefin to the reaction stage or passing it to a working up stagedownstream from the reaction stage.

[0009] In a preferred embodiment, an inert gas stream is additionallyintroduced into the absorption unit, wherein the inert gas leaves theabsorption unit together with the oxygen in the exit gas stream. Thequantity of inert gas introduced is here preferably selected as afunction of the quantity and composition of the exit gas stream leavingthe reaction stage such that the exit gas stream leaving the absorptionunit is no longer of an ignitable composition. This embodiment has theadvantage that, even in the case of variation in product streams in theoverall process, it is very simple constantly to maintain thecomposition of the gas phase in the absorption unit such that anignitable mixture cannot occur within the absorption unit, nor may itleave said unit as an exit gas stream.

[0010] Suitable inert gases are any gases which dissolve only slightlyin the solvent used for epoxidation, do not react with hydrogen peroxideand olefin oxide under the epoxidation reaction conditions and do notform explosive mixtures with oxygen. The inert gas preferably usedcomprises nitrogen or an inert gas obtained by combustion of amethane-air mixture.

[0011] Suitable solvents are any solvents which are not oxidized or areonly slightly oxidized by hydrogen peroxide under the selected reactionconditions and dissolve in water in a quantity of greater than 10 wt. %.Preferred solvents are those which are unlimitedly miscible with water.Suitable solvents are alcohols, such as for example methanol, ethanol ortert.-butanol; glycols, such as for example ethylene glycol,1,2-propanediol or 1,3-propanediol; cyclic ethers, such as for exampletetrahydrofuran. dioxane or propene oxide; glycol ethers, such as forexample ethylene glycol monomethyl ether, ethylene glycol monoethylether, ethylene glycol monobutyl ether or propylene glycol monomethylethers and ketones, such as for example acetone or 2-butanone. Methanolis particularly preferably used as the solvent. Absorption is performedat a total pressure in the range from 1 to 25 bar, preferably at thesame pressure as the epoxidation reaction, at which the exit gascontaining oxygen is obtained. Absorption may be performed attemperatures between the melting point of the solvent and 100° C.,preferably in the range from 0 to 60° C.

[0012] In a particularly preferred embodiment of the present invention,the inert gas stream and exit gas stream are passed countercurrently tothe solvent. An absorption unit which is suitable for this embodiment isin particular a column with an inert packing or inserts, wherein theexit gas stream loaded with olefin and olefin oxide and the inert gasstream are fed into the bottom of the column, the solvent is supplied tothe top of the column, the exit gas stream is discharged at the top ofthe column and the solvent stream loaded with olefin and olefin oxide isdrawn off from the bottom of the column.

[0013] The process according to the invention is suitable for theepoxidation of olefins having 2 to 6 carbon atoms. The epoxidation ofpropene to yield propene oxide is most highly preferred. The processaccording to the invention is thus illustrated below using theepoxidation of propene by way of example.

[0014]FIG. 1 shows one embodiment of the invention in which theabsorption stage is incorporated into the epoxidation process in such amanner that the solvent stream loaded with propene and propene oxideduring absorption is passed into the epoxidation reaction.

[0015]FIG. 2 shows an alternative embodiment of the present invention,in which the solvent stream loaded with propene, propane and propeneoxide is passed not into the epoxidation stage but instead into theworking up stage.

[0016]FIG. 3 shows an absorption unit suitable for the process accordingto the invention.

[0017] According to FIG. 1 or FIG. 2, propene is introduced into theepoxidation stage with stream 1, hydrogen peroxide with stream 2 and thesolvent with stream 3, wherein stream 3 serves to make good any solventlosses in the process. A liquid reaction mixture leaves the epoxidationstage with stream 4 and an exit gas containing oxygen with stream 5. Theliquid reaction mixture of stream 4 is separated during working up intostream 6, which substantially consists of propene and propane, stream 7,which substantially consists of propene oxide, stream 8, whichsubstantially consists of the solvent, and stream 9, which substantiallyconsists of water and high-boiling secondary products. When a suspendedcatalyst is used, working up also recovers the catalyst as stream 10,which is returned to the epoxidation, wherein a proportion of thecatalyst or the entire catalyst is optionally subjected to aregeneration step beforehand. When a shaped catalyst is used which isretained in the epoxidation stage, stream 10 is not obtained. Therecovered solvent from stream 8 is returned entirely or in part asstream 11 to the absorption stage. A substream 12 of the propene stream6. which is returned to the epoxidation, is passed into the propaneseparation stage, from which a propane-enriched stream 13 is discharged,while the propane-depleted stream 14 is also passed into the epoxidationstage.

[0018] An inert gas stream 15 is also introduced into the absorptionstage in addition to the exit gas stream 5 containing oxygen and thesolvent stream 11. The inert gas leaves the absorption stage togetherwith the oxygen from the epoxidation with the exit gas stream 16.

[0019] In the embodiment shown in FIG. 1. the solvent stream 17 loadedwith propene, propane and propene oxide is returned to the epoxidationstage and the solvent stream 8 is divided such that preferably more than30% and particularly preferably more than 80% are passed with stream 11into the absorption stage.

[0020] In the embodiment shown in FIG. 2, the solvent stream 17 loadedwith propene, propane and propene oxide is passed into the working upstage and the solvent stream 8 is divided such that preferably less than50% and particularly preferably less than 30% are passed with stream 11into the absorption stage.

[0021] In the absorption stage, the exit gas stream 5 and the solventstream 11 are passed countercurrently. The absorption stage preferablytakes the form as shown in FIG. 3 of a column with an inert packing orinserts and is operated such that the gas streams 5 and 15 are fed intothe bottom of the column and the exit gas stream 16 is drawn off fromthe top of the column, while the solvent is supplied with stream 11 intothe top of the column and the loaded liquid stream 17 is withdrawn fromthe bottom of the column.

[0022] The quantity of inert gas is selected such that the exit gasstream 16, which, apart from inert gas, also contains oxygen togetherwith small quantities of propene and solvent, is no longer of anignitable composition.

[0023] In the epoxidation stage, stream 2 containing hydrogen peroxidemay be mixed with a stream containing solvent (stream 8 and/or 17 in theembodiment according to FIG. 1 or stream 8 in the embodiment accordingto FIG. 2), before it is passed into the epoxidation reactor. Streams 1and 6 containing propene may likewise be mixed before they are fed intothe epoxidation reactor.

[0024] The process according to the invention has the advantage thatonly small quantities of propene and propene oxide are lost with theexit gas stream containing oxygen and that no additional auxiliarysubstances are required to recover the propene and propene oxide fromthe exit gas stream, as the solvent used for the absorption may berecirculated with the solvent used in the epoxidation reaction.

1. Process for the catalytic epoxidation of olefins in which in onereaction stage the olefin is reacted with aqueous hydrogen peroxide inan organic, water-miscible solvent in the presence of a titaniumsilicalite catalyst, wherein an exit gas stream (5) is obtained whichcontains olefin oxide, unreacted olefin and oxygen, characterized inthat this exit gas stream (5) is brought into contact in an absorptionunit with a solvent stream (11) consisting essentially of the samesolvent as used in the reaction stage and a solvent stream (17) loadedwith olefin and olefin oxide is drawn off from the absorption unit andan exit gas stream (16) containing oxygen is discharged.
 2. Processaccording to claim 1, characterized in that an inert gas stream (15) isadditionally introduced into the absorption unit, wherein the inert gasleaves the absorption unit together with the oxygen in the exit gasstream (16).
 3. Process according to claim 2, characterized in that thequantity of inert gas introduced is selected as a function of thequantity and composition of the exit gas stream (5) leaving the reactionstage such that the exit gas stream (16) leaving the absorption unit isno longer of an ignitable composition.
 4. Process according to one ofclaims 2 and 3, characterized in that the inert gas is selected from agas which dissolves only slightly in the solvent used for epoxidation,does not react with hydrogen peroxide and olefin oxide under theepoxidation reaction conditions and does not form explosive mixtureswith oxygen, preferably from nitrogen or an inert gas obtained bycombustion of a methane-air mixture.
 5. Process according to one ofclaims 2-4, characterized in that the inert gas stream (15) and the exitgas stream (5) are passed countercurrently to the solvent.
 6. Processaccording to one of the preceding claims, characterized in that theabsorption unit is a column with an inert packing or inserts and the gasstreams (5, 15) are fed into the bottom of the column, the solvent issupplied to the top of the column as solvent stream (11), the exit gasstream (16) is discharged at the top of the column and the solventstream (17) loaded with olefin and olefin oxide is drawn off from thebottom of the column.
 7. Process according to one of the precedingclaims, characterized in that the solvent stream (17) loaded with olefinand olefin oxide is either returned to the reaction stage or is passedto a working up stage downstream from the reaction stage.
 8. Processaccording to one of the preceding claims, characterised in that theliquid product stream (4) from the reaction stage is worked up and therecovered solvent so obtained is returned in part to the absorption unitand in part to the reaction stage.
 9. Process according to one of thepreceding claims, characterised in that the olefin is an olefin having2-6 carbon atoms, preferably propene.
 10. Process according to one ofthe preceding claims, characterised in that the solvent is selected fromamong alcohols, glycols, cyclic ethers, glycol ethers and ketones and ispreferably methanol.